This invention relates to the art of communications circuits, and more particularly to a new and improved multicoupler characterized by broadband, constant voltage operation.
In communication systems, for many reasons a single signal source is desired to be distributed among several utilization devices, and for this purpose a power divider is used. A typical example is a single antenna supplying a plurality of receivers. A power divider of N output ports develops 1/N of the input power at each of the output ports. When the input and output impedances are equal, the output voltage is reduced also. Fifty ohm input and output impedances generally are used to allow standard coaxial cables to be used for interconnection to other equipment. If such power divider were placed directly at the antenna output port and before a communications receiver, the voltage reduction or signal loss would add directly to the noise figure of the receiver and, in effect, degrade the sensitivity of the receiver. Therefore, standard practice is to place a preamplifier ahead of the power divider of sufficient gain so that no loss of sensitivity occurs.
One disadvantage of the foregoing arrangement is that significant demand is on proper amplifier characteristics which will not allow performance degradation in situations of high off-channel interfering signals. High gain of course increases the effect of already strong signals. The requirement of the preamplifier also undesirably complicates expansion of an existing system to a larger number of output ports, often by requiring rearrangement of components within the multicoupler, and also by requiring additional preamplifiers which, in turn, due to the increased amplification can cause already strong off-channel signals to be increased to the point of causing intermodulation and other distortion effects. Another disadvantage of the foregoing power divider is that it typically includes transmission lines which are odd multiples of one-quarter wavelength to act as impedance transformers. As a result, the frequency dependence of the transmission lines causes the power divider to have limited bandwidth.
It would, therefore, be highly desirable to provide a multicoupler wherein the voltage is substantially the same at the input and outputs such that the signal to noise ratio is substantially unaffected, which has the ability to duplicate a low level input signal at multiple output ports with no restriction on bandwidth of operation, and which can be placed directly at the signal source without the need for a preamplifier.